Corticosteroid hormones are thought to promote optimal behavioral adaptation under fearful conditions, primarily via glucocorticoid receptors (GRs). Here, we examined – using pharmacological and genetic approaches in mice – if mineralocorticoid receptors (MRs) also play a role in fearful memory formation. As expected, administration of the GR-antagonist RU38486 prior to training in a fear conditioning paradigm impaired contextual memory when tested 24 (but not when tested 3) h after training. Tone-cue memory was enhanced by RU38486 when tested at 4 (but not 25) h after training. Interestingly, pre (but not post)-training administration of MR antagonist spironolactone impaired contextual memory, both at 3 and 24 h after training. Similar effects were also found in forebrain-specific MR knockout mice. Spironolactone also impaired tone-cue memory, but only at 4 h after training. These results reveal that – in addition to GRs – MRs also play a critical role in establishing fear memories, particularly in the early phase of memory formation.
Exposure to emotionally arousing events activates the Hypothalamo–Pituitary–Adrenal (HPA) axis. As a consequence, enhanced levels of corticosteroid hormones (corticosterone in most rodents and cortisol in humans) are released into the circulation (de Kloet, Joels, & Holsboer, 2005). Corticosteroid hormones enter the brain and bind to two receptor subtypes; the mineralocorticoid receptors (MRs) and glucocorticoid receptors (GRs). MRs are occupied when hormone levels are low and exert their effects classically via transcriptional regulation of responsive genes (de Kloet et al., 2005). However, recent evidence shows that corticosteroid hormones can also exert rapid non-genomic effects via MRs (Karst et al., 2010, Karst et al., 2005 and Olijslagers et al., 2008). Compared to MRs, GRs have a 10-fold lower affinity for corticosterone, become activated when hormone levels rise after stress and slowly exert genomic actions (de Kloet et al., 2005 and Joels et al., 2006) but also non-genomic effects (Di et al., 2003 and Karst et al., 2010).
Corticosteroid hormones alter neuronal activity in areas that play central roles in attention and selection of appropriate behavioral strategies (i.e. hippocampus, prefrontal cortex and amygdala; (de Kloet et al., 2005, Joels, 2010 and Roozendaal et al., 2009). As part of behavioral adaptation to stressful events, these hormones via activation of MRs and GRs, in interaction with other hormones and neurotransmitters (Joels and Baram, 2009 and Roozendaal et al., 2009), promote the storage of information (McEwen and Gianaros, 2010 and Oitzl et al., 2010).
Corticosteroid hormones in vitro rapidly increase neuronal activity in the hippocampus (Karst et al., 2005 and Olijslagers et al., 2008) and amygdala (Karst et al., 2010) via a mechanism that requires MRs activation. Behavioral studies indicate that MRs are involved in appraisal of information and response selection in various tasks (Brinks et al., 2007, Oitzl and de Kloet, 1992, Oitzl et al., 2010 and Sandi and Rose, 1994a). Moreover, genetic deletion of MRs in the forebrain led to various cognitive impairments, including impaired learning in a Morris water-maze task (Berger et al., 2006). In contrast, via the activation of GRs, a gene-mediated cascade is initiated to slowly restore neuronal activity and suppress synaptic plasticity (Joels and de Kloet, 1989, Kerr et al., 1989 and Kim and Diamond, 2002). At the behavioral level, activation of GRs promotes consolidation of the acquired stressful information at the later phase of memory formation (Lupien and McEwen, 1997, Oitzl et al., 2001, Pugh et al., 1997a, Pugh et al., 1997b and Roozendaal, 2003).
The existing data on the role of MRs in neuronal activity and cognitive function justifies the question whether MRs also play a role during the early phase of memory formation, in addition to the role of GRs in promoting memory formation at later time points (e.g. up to 1 day after acquisition of information). We therefore tested this hypothesis by examining whether specific blockade of MRs and GRs interferes with contextual and tone-cue memory formation at two different time points (i.e. contextual memory tested either 3 or 24 h after training, tone-cue memory tested 1 h later, i.e. either 4 or 25 h after training). These time points (3–4 h versus 24–25 h) presumably reflect different learning phases (early encoding versus long-term memory) and cellular events that may underlie the learning process (Zhou, Conboy, Sandi, Joels, & Krugers, 2009). Corticosteroids have been reported to particularly modulate synaptic plasticity evoked by weak stimulation paradigms (Alfarez et al., 2002 and Pu et al., 2009) and promote transition from short- to long-term memory in a relatively weak aversive learning paradigm (Cordero and Sandi, 1998 and Sandi and Rose, 1994b). We therefore tested the roles of MRs and GRs both on contextual and tone-cue memory formation using mild (less aversive) and relatively strong (more aversive) learning paradigm, with shock intensities of 0.4 and 0.8 mA respectively.
